The present invention relates to DC voltage power supplies and more particularly to inrush/transient current limiting and overload/short circuit protection for DC voltage power supplies.
Many applications that utilize DC voltage encounter various operating conditions that could potentially damage electrical system components. Providing protection against damage during these operating conditions can pose difficult challenges for DC voltage power supplies. In some applications, such as aerospace systems, operating situations that can cause damage include inrush currents, transient currents, circuit overloads and short circuits. Inrush currents may consist of a momentary high current that occurs when the power supply is first turned on. Transient current increases may occur due to brief changes in the input voltage. Circuit overload may be caused by various malfunctions occurring in the loads, and short circuits may occur when feeder lines are shorted.
FIG. 1 shows a typical DC power system 100 in accordance with the prior art. A DC power source 102 includes a DC source Vin 104, a source resistance Rs 106 and source inductance Ls 108. In an exemplary application, such as avionics, DC power source 102 may be either 28V or 270 V. The DC power source 102 may be connected to a switch 110 and to wiring 112, which has wiring resistance Rw 114 and wiring inductance Lw 116. A load 118 may be connected across the positive and negative terminals of the DC power source 102. Load 118 may include a load capacitance 120 and a load resistance 122. The switch 110 may be used to turn on and off DC power to the load 118 and may comprise, for example, a circuit breaker, a relay or an electronic switch.
In operation, the DC voltage Vin produced by the DC power source 102, may vary widely over short periods of time. These transient variations may be due to a number of occurrences. For example, if DC source 104 is a battery, DC voltage variations may be the result of the charging and discharging of the battery. Load capacitor 120 may provide stabilization of load voltage during these variations in Vin. When the switch 110 is turned on, the source voltage Vin may charge the load capacitor 120 through the DC source 102 and wiring 112 impedance. Since Cload is high, and usually Rs, Ls, Rw, and Lw are all small, the inrush current may be relatively large. A large inrush current may be harmful to the DC power system 100 by stressing the power source and the switch and also by producing electromagnetic interference (EMI).
Inrush current occurs not only at power on, but can occur due to the aforementioned transient variations in input voltage Vin. For example, when a step change is applied in the input voltage from minimum to maximum voltage, the induced inrush current can be large according to the slew rate. Hence there is a need to limit inrush currents and transient currents.
Short circuit protection is also needed for DC power systems. In the event of a short circuit condition a switch, such as switch 110, should be turned off fast enough before any damage can occur. At the same time, there should not be any nuisance trip of switch 110 under normal operating conditions. In avionics systems it is desirable for a circuit providing such short circuit protection to be relatively simple, low cost, and to be able to operate over a wide input voltage range and a wide temperature range.
FIGS. 2 and 3 show circuits 200 and 300 respectively, in accordance with the prior art. Circuits 200 and 300 each have simple R-C circuits that are used to extend the rise time of the load voltage and thereby limit inrush current. In FIG. 2 a switch 202 may be placed on the low side along with an R-C circuit 204 that may include resistors 206 and 208 and a capacitor 210. During power up the capacitor 210 voltage may be charged slowly, which will turn on switch 202 gradually from off to on. During power up, switch 202 may be going through a linear operation area for a pre-determined period of time defined by the parameters of resistors 206 and 208 and capacitor 210. As a result, the output voltage Vout of the circuit may increase with a relatively slow changing rate, which will limit the inrush current level.
FIG. 3 shows a circuit 300 that is similar to circuit 200 except that it may include a switch 302 placed on the high side. Circuit 300 may include an R-C circuit 304 having resistors 306 and 308 and a capacitor 310. Both circuits 200 and circuit 300 may only be effective in limiting inrush current during power up and may not provide inrush protection for transient inrush currents.
FIG. 4 shows a circuit 400 that may be commonly used for inrush current limit in accordance with the prior art. In circuit 400 the inrush current may be initially limited by a resistor 402. After a transient period, a switch 404 may be closed to short resistor 402 out of the current path. Control logic unit 406 may be used for switching control. Switch 404 may comprise contactor or electronic switches, for example, a metal oxide semiconductor field effect transistor (MOSFET), or insulated gate bipolar transistor (IGBT), etc. Where switch 404 is a contactor, the circuit 400 may be only useful for inrush current limit during power up and may not be able to achieve inrush current limit caused by input voltage transients after power on. Overload and short circuit protection functions also may not be achieved due to the slow response of contactor.
Where switch 404 is an electronic switch the circuit 400 may have fast dynamic response. However, in order to achieve transient current limit and overload/short-circuit protection functions, control circuit 406 will need to include complex driver circuitry and fast control logic, which will increase the circuit complexity, cost and circuit board area.
FIG. 5 shows a current limit circuit 500 in accordance with the prior art. Current limit circuit 500 may include MOSFET switch 502 and a transistor 504 having its collector connected to the gate of MOSFET switch 502. Resistor 506, 508, 510 and 512 may be connected as shown in FIG. 5. In operation, circuit 500 may have MOSFET switch 502 operating in the linear mode. When the load current reaches a predetermined limit setting, the transistor 504 may be on, which may pull down the gate voltage of MOSFET switch 502 so that the load current stays at the limit setting. The MOSFET switch 502 may be operating in linear mode, because the gate voltage is low, and both the drain current Id and the drain to source voltage Vds of MOSFET switch 502 are high.
The circuit 500 in FIG. 5 may limit any kind of inrush current, whether due to power up, or input voltage transient. However, the current limit level in this circuit is quite sensitive to environmental temperature and input voltage level. This is because the current limit level may be set by the transistor 504 base to emitter voltage Vbe. Unfortunately, Vbe changes with the environmental temperature. Collector current Ic of transistor 504 may also change if the input voltage varies. Hence, circuit 500 may not be suitable for applications having wide temperature and input voltage variations, such as aerospace applications. In addition, as with circuits 100, 200, 300 and 400, circuit 500 may not provide overload and short circuit protection functions.
As can be seen, there is a need for a circuit for DC voltage power supplies that can provide inrush current protection upon startup and during input transients. There is also a need for such a circuit that can provide fast short circuit protection and overload protection. There is also a need for a circuit with the aforementioned features that is relatively simple, low cost, and able to operate over a wide input voltage range and a wide temperature range.